1. Field of Invention
This invention relates to apparatuses for controlling the feed of machine tools and more particulary to apparatuses for controlling the feed of machine tools which employ a DC motor.
2. Prior Art
Generally, the table which moves the main shaft or work table, etc., of a machine tool into prescribed positions is driven by a hydraulic cylinder system. Hydraulic cylinder systems have several advantages, such as being relatively easy to control, being able to produce a sizeable driving force, etc., which make them suitable for generating thrust for feed tables. However, due to various factors such as temperature fluctuations and use of compressable fluids, etc., feed control stability is poor. This poor stability in feed control leads to decreased productivity and shorter tool life. Furthermore, these systems also suffer from other drawbacks, including a number of environmental problems such as oil leaks, noise, etc.
Systems in which a motor drive is used to move a table have been proposed as an alternative to such conventional hydraulic cylinder systems. These motor driven systems, however, do not have all the advantages offered by conventional hydraulic cylinder systems. Particularly in the case of AC motor driven systems, it is difficult to change the rate of revolution of the motor. These systems have therefore had to employ complicated mechanisms such as gear boxes, etc. Such complicated mechanisms have increased the cost of such systems. Furthermore, in cases where the table is stopped in prescribed positions, it is difficult in these AC motor driven systems to hold the table securely in its stopped position. Accordingly, these systems have suffered from several drawbacks, such as poor precision in returning the table from the forward end, shortened tool life due to mechanical abrasion, etc.
The use of DC motors is desirable from the standpoint of ease of rate control, but these prior art DC motor driven systems suffer from the same drawbacks as the previously described AC motor driven systems in regard to their table holding action.
This invention has been designed with the above described conventional problems in mind.